This invention relates to a polishing method usable for polishing the surface of a target object such as a semiconductor device wafer, a magnetic hard disk or its substrate (such as an aluminum substrate and a glass substrate), and an optical component such as a glass lens, a prism and a reflective mirror that requires a high level of flatness on the surface. In particular, this invention relates to a method of polishing for the planarization of the surface of a thin film such as an oxide film and a metallic film formed on a wafer during the production of a semiconductor device.
The surface of an object, such as a semiconductor device wafer, a magnetic hard disk or its substrate (such as an aluminum substrate and a glass substrate), and an optical component such as a glass lens, a prism and a reflective mirror that requires a high level of flatness on the surface, is usually mirror-polished by using slurry having abrading particles dispersed within a liquid, while there are situations where this kind of polishing process is carried out after a rough polishing process referred to as the lapping process.
In the above and throughout herein, the polishing shall broadly mean the process of pressing the target object to be polished onto a polishing means or the polishing means onto the target object and rubbing them together one against the other. Both the mirror-polishing process and the lapping process mentioned above are considered a kind of the polishing process thus broadly defined.
The types of polishing include both the fixed particle polishing and the free particle polishing.
The fixed particle polishing is a process by using polishing means having abrading particles dispersed and fixed in a synthetic resin material and is an effective method exhibiting high fabrication efficiency against materials that are hard to fabricate. The fixed particle polishing is generally used in the lapping process as a preliminary process prior to the polishing.
Porous polishing pads having abrading particles dispersed and fixed are included in the polishing means having abrading particles fixed in a synthetic resin material.
Examples of such porous polishing pad include, for example, those having abrading particles dispersed and fixed in a woven or unwoven sheet and those having abrading particles dispersed and fixed in a planar material comprising a foamed substance (or a foamed sheet).
Examples of polishing means having abrading particles fixed in a synthetic resin material as described above also include polishing pads having abrading particles dispersed and fixed in a planar material of a non-foamed material (or a planar uniform, solid material of a synthetic resin) (or a non-foamed sheet).
In a fixed particle polishing process, a polishing liquid is caused to be present between the target object and the polishing means when the surface of the target object is polished, and examples of such polishing liquid include slurry having abrading particles dispersed within a liquid, a cooling or lubricating liquid not containing abrading particles, liquids containing an agent that reacts chemically with the surface of the target object, and slurry having abrading particles dispersed in such a liquid.
The free particle polishing is a process whereby a polishing liquid containing abrading particles is caused to be present between the target object and the polishing means for polishing the surface of the target object. This process is generally used as the finishing process, and examples of polishing liquid containing abrading particles include slurry having abrading particles dispersed within liquid and such slurry having added thereto an agent that reacts chemically with the surface of the target object.
A porous polishing pad not having abrading particles fixed is used as the polishing means, and example of such porous polishing pad include woven and non-woven sheets and foamed pads. Examples of polishing means usable in a free particle polishing process further include non-foamed pads such as polishing pads comprising a uniform and solid planar material made of a synthetic resin.
The surface of a target object requiring a high level of flatness (such as semiconductor device wafers) is thus polished by a free particle polishing process.
Next, the polishing of the surface of a semiconductor device wafer, or its planarization, will be explained.
Conventionally, the surface planarization of a semiconductor device wafer (hereinafter referred to simply as the “wafer”) has been carried out by using a polishing device of the oscillating polishing head type (shown at 30′ in FIG. 1A) or of the oscillating platen type (not shown).
The polishing device 30′ shown in FIG. 1A is comprised of a platen P having a circular flat surface, a polishing head C for holding a wafer W and pressing the surface of this wafer W onto the surface of a porous or non-foamed circular polishing pad 20 pasted onto the surface of the platen P, an oscillating mechanism (not shown) for causing the polishing head C to undergo an oscillatory motion in the radial directions shown by arrows T of the platen P, and a nozzle N for supplying a polishing liquid containing abrading particles onto the surface of the polishing pad.
The polishing pad 20 is pasted onto the surface of the platen P such that its center point 21 will fall upon the center of rotation 31 of the platen P, while the platen P is adapted to rotate in the direction of arrow R around its center of rotation 31 by the operation of its driving mechanism (not shown) connected to the platen P.
As described in Japanese Patent Publication 2006-250205, the wafer W is kept so as to match the center of rotation of the polishing head C within an annular retainer ring provided on the lower surface of the polishing head C and rotates in the direction of arrow r around the center of the wafer W, or the center of its rotation 32 by the operation of its driving mechanism (not shown) connected to the polishing head C.
In order to make uniform within its surface the relative speed (with respect to the polishing pad 20) of the wafer W pressed against the polishing pad 20, the platen P and the wafer W are rotated in the same direction (as shown by arrows R and r).
A circular foamed or non-foamed member or a non-foamed pad with abrading particles not fixed is usually used as the polishing pad 20 pasted to the surface of the platen P, and, as shown in FIG. 4, grooves 22 are formed in the form of concentric circles around the center 21 of the polishing pad 20 passing through the center of rotation 31 of the platen P on the surface of the polishing pad 20 for stabilizing the flow of the polishing liquid supplied to the surface of the polishing pad 20. As described in Japanese Patent Publications 2004-140130 and 2006-068853, for example, lattice-shaped grooves 13b as shown in FIG. 3A and dot-like holes 13a as shown in FIG. 3B may also be formed on the surface of the polishing pad 20.
In the case of a polishing pad with grooves thus formed on its surface, if the grooves are formed in a regular pattern, effects of this pattern appear on the surface of the wafer W as polishing marks. In order to reduce these effects of the groove pattern, it has been a common practice to cause the polishing head C or the platen P to undergo a reciprocating motion in radial directions of the platen P as shown by arrows T, as described, for example, in Japanese Patent Publication 2006-068853. This reciprocating motion is generally referred to as oscillation. In the illustrated example, it is the polishing head C that is being oscillated in the directions of arrows T. The width of this oscillation is generally set within one pitch of the groove pattern formed on the polishing pad, and the oscillatory motion is usually carried out with frequency in the range of about 0.01 Hz-0.04 Hz and the waveform controlled sinusoidally or trapezoidally.
This is the way the surfaces of semiconductor device wafers are being polished, but the surfaces of target objects of other kinds described above (such as magnetic hard disks and their substrates) are also being polished by using a polishing device of the oscillating polishing head type or of the oscillating platen type, as described above, that is, by moving the polishing head or the platen oscillatingly during the polishing process.